The use of powder coating methods to provide tough, abrasion-resistant finishes is well known, as described, for example in Kirk-Othmar, Encyclopedia of Chemical Technology, third Edition Vol. 19, pp. 1-27, (1982). Coating powders are characterized by very low organic emissions during powder coating processes and therefor have important environmental, health and safety advantages relative to other types of coatings.
There are circumstances where it would be highly desirable to powder coat a substrate with a metal-containing coating powder so as to provide a coating with a highly reflective, metallic appearance or to provide a sparkle finish. In the area of decorative coatings, striking effects can be achieved through metal-containing coatings. Highly reflective coatings are useful in providing identification and easy recognition of objects. Metal-containing coatings may be used to lower the temperature of vessels, provide solar reflectivity, etc. The widespread utility of conventional solvent-based paints using leafing aluminum pigments clearly illustrates the utility and desirability of metallic coatings. To date, metal-containing coatings applied by powder coating processes have not achieved the luster provided by solvent-based paints, and/or, it has proven difficult to maintain uniformity of coating.
When flat, pancake-like metal flakes, such as "leafing aluminum" are used in a solvent-based paint, the metal flakes tend to align parallel to the surface of the coating, thereby presenting flat, reflective surfaces which give a metallic appearance to the coating when the paint dries or is baked. Similar results have been difficult to achieve in powder coatings. It is known to integrally incorporate metal flakes into the coating powder by admixing the metal flakes with the resin, flow-control agents, curing agents, pigments, fillers, etc., prior to melt-compounding of the ingredients. However, during grinding of the melt-compounded composition to produce a coating powder, the flakes are very significantly fragmented, and the finish that results from such a coating powder has a dull, grey appearance.
U.K. Patent Specification 1,404,556 published Sep. 3, 1975 describes coating powders in which aluminum flakes are imbedded into the powder by milling, e.g, in a ball-mill. Again, the flakes are very significantly fragmented. Coating powders produced by this method do not achieve the luster of comparable solvent-based metallic paints.
U.S. Pat. No. 4,197,351 to Rolles et al. describes brush polishing a dry mixture of metal flakes and plastic powder so as to embed the flakes into the powder. Although the coating powder obtained may have a high luster, the brush polishing method described in this patent is impractical for industrial scale production of coating powders.
The most straight-forward method of obtaining a metallic finish from a powder coating process is to simply admix metal flakes with the polymeric coating powder particulates. However, in the powder coating process, as is actually performed in industrial settings, such an admixture results in inconsistent coatings. In electrostatic coating, coating powder is sprayed through a gun where a high voltage corona discharge is used to establish an ionized field. As the powder particles pass through the ionized field, they become charged and are attracted to the substrate, which is usually a metallic article to be coated. The powder is subsequently fused to form a continuous film. In the spraying process, any powder which is not deposited on the substrate is collected in a reclaim system and returned for admixture with the virgin material for reapplication. When metal flakes are merely admixed with the polymeric coating particulates, the flakes are not charged to the same extent or deposited at the same rate as the polymeric coating powder particulates. Over a period of time during which overspray is continuously reclaimed and admixed with virgin material, the metallic flake pigment becomes more concentrated in the powder being sprayed, changing the appearance of the resulting finish from the finish which resulted from spray at the start of the run. Eventually, the higher concentration of metal flakes may interfere with the charging mechanism. If the powder in the ionized field becomes too conductive, the electrostatic gun may "short out".
There remains a need for a practical method to produce coating powders which result in a lustrous metallic finish that is consistent over time in a process in which overspray particles are reclaimed and returned. There also exists a need for more uniform coating (over time) from coating powders containing non-metallic flakes, such as mica flakes.